1. Field of the Invention
The invention relates to a method and to a device for determining the thickness of an electrically conductive protective layer of a component of a turbine system. The protective layer has an electrical conductivity .kappa..sub.1 and is applied to a base material having an electrical conductivity .kappa..sub.2. The electrical conductivities .kappa..sub.1 and .kappa..sub.2 are different from each other.
Various methods for determining the thickness of a corrosion protection layer for a gas turbine blade are described in the article titled "Non-destructive Testing of Corrosion Effects on High-temperature Protective Coatings" in VGB-Kraftwerkstechnik 70 (1990) No. 9, pages 645 to 651, by G. Dibelius, H. J. Krichel and U. Reimann. One of the methods described is the so-called "eddy-current measurement method", in which the different electrical conductivities of the protective layer and of the base material are employed. Using a flat coiled copper probe which is applied to a flexible support, for example, a printed conductor, an eddy current is induced in the gas turbine blade. To this end, a high-frequency alternating electric current is applied to the probe and the impedance of the probe is recorded. For a fixed frequency of an alternating electric current, a characteristic value of the impedance results as a function of the thickness of the layer, the material of the layer and the material of the base material. In the article, the thickness of the layers were examined as a function of the impedance for a protective layer made of a platinum-aluminum alloy on a base material, an IN 738 LC stainless steel. The impedance of the layers having a thickness of up to 1 mm were measured at a frequency between 200 kHz and 500 kHz. However, the article does not contain information regarding either the values or the ratio of the electrical conductivities nor information regarding the accuracy and reproducibility of the measurement of the gas turbine blades with an unknown layer thickness for determining the thickness of the layer.
A method for determining the thickness of a zirconium coating on the inside of a zirconium alloy tube used in nuclear reactor fuel elements is described in Non-prosecuted, German Patent Application No. 33 35 080 A1. The determination method uses the eddy-current testing principle, in that the impedance variation of a coil is evaluated by a high-frequency eddy-current field induced in the zirconium coating. The frequency for the high-frequency eddy-current field is selected in such a way that an impedance variation attributable to the so-called lifting-off of the excitation coil can be discriminated clearly from an impedance variation because of the thickness of the layer. Frequencies suitable for this purpose are in the range between 6 MHz to 20 MHz. The thickness of the zirconium layer extends from 10 .mu.m to about 100 .mu.m, the resistivity of the zirconium layer is approximately 40*10.sup.-8 .OMEGA.m, and the resistivity of the base metal, the zirconium alloy, is approximately 74*10.sup.-8 .OMEGA.m. The conductivity of the zirconium layer is therefore about twice the conductivity of the base metal. Using the described eddy-current testing method for applications in the nuclear technology field, the intention is to be able to carry out a determination of the zirconium layer thickness with a deviation of about 5 .mu.m accuracy.